This invention generally relates to electrochemical microsensors and methods for their assembly.
Specific, and often independently addressed, areas of electrochemical microsensor packaging and encapsulation are: electronic isolation from solution; lead attachment and encapsulation; and membrane attachment and isolation. Several solutions have been found for each individual problem area, but these often complicate the sensor fabrication process, making the process non-compatible with standard integrated circuit processing techniques. The resulting development and manufacturing costs are exorbitant, particularly for a multi-sensor device.
Chemical field effect transistors, or CHEMFETS, provide a means for sensing the concentration of chemical ions. The basis of operation of and FET is that a field is induced in the channel region which controls the conductivity between source and drain. Since the gate of a CHEMFET includes the surrounding solution, as well as any electrodes which bias the solution, isolation from the solution is as important to their proper functioning as is the isolation of MOS components and circuits from one another and from the environment. In addition, the surrounding solution is of changing chemistry, sensed by a change in gate potential. All interfaces with the solution have a characteristic potential which can alter interfacial processes, i.e., exchange currents, therein. Since FET fabrication is most readily accomplished using planar technologies, CHEMFET fabrication processes have traditionally employed solid state coatings with low water and ion permeability, such as silicon nitride and aluminum oxide, on the upper surface. Chemical vapor deposition (CVD) and patterning techniques for high-quality materials have been developed and perfected by the integrated circuit industry. However, processes must take into account that the substrate is exposed at the sides of the die after separation of the individual die from the silicon wafers. One way in which isolation of the exposed substrate can be accomplished is by encapsulation after die separation with an impermeable insulating material such as an epoxy resin or silicon nitride. More recently, diode isolation and SOI (silicon-on-insulator) techniques have been applied to the problems of substrate isolation.
Chemical microsensors, especially active devices such as the CHEMFET, require electrical signal and power supplying leads which communicate between the device in solution and the outer dry data acquisition environment. A means is therefore required by which leads can be attached to the chip and still be electrically isolated from one another and the ambient environment. Encapsulation of the leads from water and ions in the solution is required to prevent corrosion. Several approaches have been explored including commercially available bonding techniques such as wire bonding, tape automated bonding, etc. The physical bonding of the leads to the chip has not been as significant a problem as the difficulty in how to encapsulate the bonds and lead wires without covering the sensitive gate region. In one solution, the bonding pads for the CHEMFETs are placed along one edge of the chip, as far as possible from the active gate area.
Unfortunately, there are no suitable, commercially available chip carriers or cables for chemical microsensors. Printed circuit cards and dual lumen catheter are among the hand fashioned chip carriers and cabling that have been employed. While TAB bonding with commercially available Kapton.TM. tapes can reduce this problem, the adhesion layer fails the long-term exposure to ionic, aqueous solutions.
Other methods of lead attachment and encapsulation can be used. Photolithographically patterned material such as Riston.TM. can be used to protect the gate regions prior to wire bonding and encapsulation. Alternatively, back side contacts, which involve etching via holes through the wafer, diffusing dopants through the entire substrate by thermal gradient, or using SOI fabrication techniques, can be used. These methods place the lead attachment and encapsulation points on the back of the chip which need not be exposed to solution if an appropriate cell design can be employed. However, this has significant limitations: the complexity of processing the combination of back side contacts and on chip electronics (substrate) isolation; the limitations on the number and placement of i/o leads due to the large space required per contact and/or poor special resolution capabilities; the difficulties encountered in making IC fabrication compatible with back side contact formation; and the problem of in vivo sensors where both the chips and the leads are immersed in electrolyte.
Generally speaking, electrochemical microsensors either sense a potential, using selectively sensitive materials as transducers of chemical energy, or they measure the rate of a reaction, represented by the current. CHEMFETs are made chemically sensitive to a specific ion or other chemical species by attaching a sensing membrane material in series with a gate insulator.
It is crucial that the membrane integrity, adhesion and isolation from other sensing gates be maintained for each sensor. Since the chemically established membrane potential is effectively in series with any applied gate bias, it is sensed in the same manner as a change in the gate voltage of a FET. Any electrical shunt path, either vertical or horizontal, through or around the membrane/solution potential generating interface, will diminish the potential sensed by the FET or other active electronic device. Horizontal shunts between membrane covered FETs will create mutually dependent sensors and diminish their sensitivity.
A number of solutions to the problem of maintaining membrane integrity, adhesion and isolation have been proposed. The original approach was to use hand painted epoxy wells with silanization of surfaces to promote adhesion. It soon became apparent that this was not a commercially viable technique. A technique tried for membrane well fabrication which proved to not be commercially feasible is the Riston.TM., E. I. Dupont de Nemours & Co., Wilmington, DE, masking technique, described by Ho, et al., in "Encapsulation of polymeric membrane-based ion-selective field effect transistors", Sensors and Actuators 4,413 (1983). Attempts to improve adhesion of subsequently solvent cast polymeric membrane materials included using microfabricated meshes made in spun cast polyimide films or membranes formed by spin casting of a plasticized polymeric matrix onto a wafer containing FETs or thin film electrodes, followed by local doping of the organic film with an ion sensitive material such as an ionophore.
Unfortunately, all of these techniques preclude the use of liquid membrane materials and the fabrication in miniature of a classical ion sensing electrode (ISE) (i.e. membrane/filling solution/redox couple/metal/amplifier). ISE-like devices are desirable since each interface in the electrochemical system is thermodynamically well defined and could therefore be fabricated in such a way as to minimize drift and enhance reproducibility. Conventional CHEMFETs have a semiconductor/ insulator/ ionic solution or semiconductor/ insulator/ membrane/ ionic solution gate structure. The coupling between ionic and electronic conduction in these systems is unclear. The insulator/membrane interface is blocked, impermeable to membrane charge transfer, and therefore not thermodynamically well defined. Modifications of CHEMFET structures to include an inner reference solution and redox couple between the membrane and gate insulator could result in a highly improved chemical microsensor.
CHEMFETs were first modified to include and inner reference solution and redox couple between the membrane and gate insulator in 1978, as described by Comte and Janata, "A Field Effect Transistor as a Solid State Reference Electrode", Anal. Chim. Acta. 101, 247 (1978). They laboriously pasted individual capillary tips over the gate region of CHEMFETs, epoxied them in place and then filled them with a pH buffer solution, entirely by hand, to create a reference FET.
In 1985, Prohaska described surface micromachining silicon nitride microchambers to form thin film, nearly planar, electrochemical cells ("New Developments in Miniaturized Electrochemical Sensors", Technical Digest, International Conference on Solid-State Sensors and Actuators, Philadelphia, PA, June 1985, pp. 401-402).
Micromachining of chambers in Pyrex.TM. glass plates using etching and laser drilling was described in two more recent papers by Blennemann, et al., Transducers '87, The Fourth International Conference on Solid-State Sensors and Actuators, pp. 723-725 and "Glass Encapsulation of Chemical Solid-State Sensors Based on Anodic Bonding", van den Vlekkert, et al., Transducers '87, pp. 730-733 (1987). The micromachined structures were anodically bonded to the FET containing substrate at temperature typically of around 400.degree. to 600.degree. C. with voltages of around 600 to 1,000 volts.
The process using anodic bonding of machined Pyrex.TM. structures has a number of drawbacks, principally related to the use of the anodic bonding. Care must be taken to avoid damaging the electronic and sensor components during bonding due to large, electrostatic fields and heat. Another problem is the requirement for a planar substrate surface, of less than 1000 angstroms steps, to insure uniform bonding between the Pyrex.TM. structure and the silicon substrate. Conventional pattern transfer techniques do not produce a surface sufficiently smooth to insure proper bonding. Further, the material which is bonded to the substrate is limited to a material having essentially the same coefficient of expansion as the substrate so that the materials do not separate upon cooling following the bonding process. Some of these problems are discussed by both Blennemann at p. 723 and by van den Velkkert at p. 730.
It is therefore an object of the present invention to provide electrochemical microsensors which can use liquid membrane materials which can be fabricated with structures functionally equivalent to classical ion sensing electrodes.
It is further object of the present invention to provide methods for assembly and packaging of electrochemical microsensors which do not require high temperatures.
It is another object of the present invention to provide electrochemical microsensors whose chemical transducing structures and their containment can be fabricated independently from any passive or active electronic devices.
It is a further object of the present invention to provide electrochemical microsensors having a variety of three-dimensional forms and variability of function.
It is still further object of the present invention to provide electrochemical microsensors having a geometric configuration which avoids problems with wire bond encapsulation.
It is another object of the present invention to provide electrochemical microsensors containing liquid electrolyte, selective and responsive biological or labile materials, and methods for their manufacture.